If fellow Australian's are wondering how the prices are so cheap, they are wholesale prices. Other costs to include are a case, LCD, power supply (I want to make a DC-DC one probably the MK2.6 on Sproggy's website.)
So I think it is looking like around $1000 for a complete system.

How does this compare with the cost others have spent? Have I missed anything else?

Well considering the AU to US exchange rate, my prices don't look too bad, granted i still got other stuff to find prices for. And I also forgot, those prices should have +10% for GST

But on the up side, these are brand new parts much of the stuff with warranty. I went to pcrecyclers.net and added up some older parts and it's not much cheaper, and the old parts aren't exactly what i want (no onboard sound etc) and i don't think they would have much warranty as well.
I just converted 531 US dollars with Yahoo, it came to about 950 AU, so my prices are pretty much inline.

rocket - basically I would eventually like to do this as a business, so I need something for people to see, and I figure people won't want to buy old secondhand parts with no warranty... not to mention the cost difference of an old system to a new one is not all that much!

You might say "but you can get a cheap old pentium for a few hundred" Yes, but not in the amount to actually run a business off.

We also have plans to make the computer a total in-car computer that logs info on the car like fuel, revs, speed etc.

Also going by the previous posts, I'm spending only the same amount as money as a few other people who've made Mp3Boxes, only the exchange rate makes it look like a lot more.

Catfish, have you build and install a unit in your car? Before thinking of getting on a commercial level first get things working in your car and you will see that there is so much involved in a project like this...
Its not about getting your home desktop and putting it in your car... it doesnt work that way...
Im sure many people here thought of starting a business involving mp3 technology in a cars and i know people who been working on it in past few years (here in Australia), so doing this sort of price count is not realistic at all...
i personally spent AU$22.80 (+ US$20 for the LCD) on whole set up, but on commercial level all this calculations are worthless!